1. Field of the Invention
The present invention generally relates to a method and apparatus for preventing electrostatic adhesion of plastic transparencies having an image formed thereon by an electrophotographic process. More particularly, the invention relates to a method and apparatus for determining and applying a charge which eliminates uncompensated charge of toner forming the image.
2. Description Of The Prior Art
Electrophotographic copying machines are now being employed to transfer images to plastic sheets for use as transparencies with overhead projectors. Typically, for the electrophotographic copier and similar electrophotographic image forming apparatus, visible patterns are produced from a latent image, formed by exposing a sensitive layer to light, which is then made visible by a dry-process technique.
In general, the sensitive layer is usually a photoconductor which is charged. On exposure, the latent image is formed as a pattern of charge and then made visible during development by having finely divided powders attracted electrostatically to the charged areas of the latent image.
Development of the image involves the attraction of small, oppositely charged particles to the electrostatic image configuration of the sensitive layer. Such small particles, usually charged, are referred to as toner. The developed image is then transferred to copy paper or a plastic sheet, if a transparency is being produced. This image transfer is facilitated by charging the copy paper with a charge opposite to that of the toner, for example by a transfer corona wire, so that toner is attracted away from the sensitive layer to the copy paper.
After the toner image transferred to the copy paper has been developed, such as, for example, by fusing or the like, the charge of the copy paper, used to attract the toner, is neutralized by application of an opposite charge. However, the oppositely charged toner is not neutralized which results in a small, but uncompensated residual charge.
When employing such an electrostatic photographic process to produce transparencies for use with overhead projectors, the images are usually formed on plastic sheets, such as PET sheets, which are treated to have a surface resistivity in the range of 2.times.10.sup.10 to 1.times.10.sup.13 ohms/sq As a result, the uncompensated toner charge gradually spreads over the surface of the transparency due to this surface conductivity after the symmetrically induced transfer charge of the transparency has been removed at the exit of the copier by the usual means.
A significant problem results in that the transparencies electrostatically stick together and are hard to separate when needed for display. This problem can persist for many hours, and even days.
Prior art attempts to address this problem typically involve providing the transparency with an antistatic layer or antistatic agent. Such transparencies are disclosed by, for example, U.S. Pat. Nos. 3,854,942, 4,070,189, 4,071,362, 4,415,626, 4,370,379, 4,480,003, 4,526,847, and 4,320,186. However, the provision of an antistatic layer or the application of an antistatic agent results in an increase in cost of the transparencies.
As noted above, the electrostatically charged copy paper is typically neutralized, for example, by a discharge electrode such as disclosed by U.S. Pat. Nos. 4,579,441 and 4,739,363. While effective in neutralizing the symmetrical charge of the photocopy sheet, such discharge electrodes make no provision for measuring or removing the residual charge of the toner forming the image thereon. Additionally, it is known in multi-color photographic image copying processes to reverse charge the medium after a toner of a first color has been applied to neutralize trapped charges and substantially revitalize the medium as disclosed by U.S. Pat. No. 4,804,602 or neutralize incompletely developed images by the application of a reverse charge as disclosed by U.S. Pat. No. 4,318,002. In each of these arrangements, however, a set charge is applied to the medium to neutralize its charge and no sensing or determination of the residual charge of the toner on the medium is made.
Also known are arrangements for controlling the charge on a moving web, such as disclosed by U.S. Pat. Nos. 3,787,706, 3,863,108, 4,271,451, and 4,363,070. These arrangements are directed to the problem resulting from the transfer of electrostatic charges to a moving web due to friction as the web moves over rollers or the like. An electro-meter is typically provided to sense the electrostatic charge on the web and a charge of opposite polarity is applied to the web during transfer. However, these arrangements do not involve the elimination of residual charge on plastic sheets having an image formed by an electrostatic photocopier or the like.